I. Field of Invention
The present invention relates to metallized substrates and particularly microwave integrated circuit (MIC) substrates for use in hybrid microwave integrated circuits and a process for making the same.
II. Description of the Prior Art
Microwave integrated circuit technology has recently come to the forefront as a highly reproducible low cost microwave fabrication technique. With the continued and growing use of microwave technology for military applications and recently in the consumer markets, there is a need for low cost microwave integrated circuits. Moreover, there is a need for low cost metallized substrates for non-microwave devices such as multilayer integrated circuits and various other hybrid circuits.
The current technologies used to manufacture microwave integrated circuits are derived from conventional low frequency manufacturing techniques. Microwave integrated circuits can be produced in either the hybrid or monolithic form. In hybrid circuits, active devices are attached to a substrate from which passive circuitry is formed. In monolithic circuits both passive and active devices are formed in situ. The increased use of hybrid microwave integrated circuits stems mainly from the fabrication of reliable microwave semiconductor components which allow the manufacture of a large range of microwave devices. The active components are incorporated in circuits consisting of passive elements which may be in discrete or distributed form. The majority of hybrid microwave integrated circuits are fabricated with distributed elements.
Presently, two technologies, the thick film technology and the thin film technology, predominate the manufacture of metallized substrates including hybrid MIC substrates. The thin film technology involves the forming of a metallized substrates by first sputtering an adhesion promoting layer such as a titanium-tungsten alloy onto a substrate such as an alumina material. Then, a conductive layer such as a layer of gold is sputtered onto the adhesion promoting layer. Finally, a pattern is formed on the substrate by means of photoresist and etching techniques. While the thin film technology can produce a metallized substrate with excellent line resolutions, it has a number of disadvantages. One of the major disadvantages is the waste of materials. Thus, in order to produce a metallized substrate, a large amount of material must be used, but only a fraction is actually deposited onto the substrate. The remainder of the sputtered material coats the walls of the vacuum chamber.
A further disadvantage of the thin film technology is that the substrates are made in a batch process. This can be a problem since one cannot be sure of the quality of the metallization until the entire batch is finished.
IEEE Journal of Solid State Circuits, Vol. Sc-5, No. 6, December 1970, "Microwave Integrated-Circuit Technology-A Surrey", Caulton, pp. 292-303. and IEEE Transactions On Parts, Hybrid, and Packaging, Vol. PHP-10, No. 2, June 1974, Foster et al, "Thick Film Techniques for Microwave Integrated Circuits" pp. 88-94 describe thin film techniques for manufacturing metallized substrates and particularly an MIC substrate in great detail and discuss its disadvantages.
The thick film technology offers an alternative to thin film technology. A metallized substrate, for example, an MIC substrate is manufactured by the thick film process by screen printing a thick film conductor composition such as a thick film gold. Generally, the thick film composition will have a glass frit base. While the thick film process is less costly than the thin film process, it too has its disadvantages.
The primary disadvantage of the thick film process is that it can only produce conductor lines and conductor line patterns with limited accuracy. Consequently, signal losses are high and as a result some devices cannot even be fabricated by this technique. In addition, the thick film technique utilizes glass frit containing pastes. As a result of the presence of the glass frit in the conductive layer, the circuit suffers high frequency losses. This may be due to the fact that glass absorbs the microwaves. In order to overcome this disadvantage, recently, there has been some experimentation with fritless inks. Fritless copper inks have recently been developed for microwave use to overcome the disadvantages of frit containing inks. While these fritless copper inks overcome the disadvantage possessed by the glass frit containing inks, the process for producing substrates with these inks is much more complicated and expensive. Thus, the firing process needs to be modified since a non-oxidizing atmosphere such as nitrogen is required. Copper will, of course, oxidize in a oxygen atmosphere resulting in a non-conductive layer. Other attempts have been made to overcome the disadvantages of the thick film and thin processes including the use of a metallo organic gold composition to form at least a portion of the metallized conductor line followed by the electroplating of a copper conductor. These attempts, however, have resulted in inferior substrates because of the poor adhesion of the electroplated copper to the metallo organic gold.
The present invention overcomes the disadvantages of the prior art techniques for manufacturing metallized substrates and particularly microwave integrated circuit substrates.